#include<stdio.h>
#include<math.h>
int main()
{
	double a, b, c;
	scanf("%lf %lf %lf", &a, &b, &c);
	double p, S;
	p = (a+b+c)/2;
	S = sqrt(p*(p-a)*(p-b)*(p-c));
	printf("%.2lf", S);
	
	return 0;
}
